A lot of this is going to be dependent on the thickness of the
copper you are cutting. I have, in times past, resorted to actually
using a sharp cold chisel and made the cuts on a large pad of mild
steel. It scars up the steel pad but worked fairly well on the
copper. I cut very near to but not actually on the layout lines on
the waste side of the copper. Put the chisel straight up and down
and smack it soundly with a heavy hammer. Be sure that the cold
chisel is quite sharp and that the temper isn’t gone out of it.
After doing the main cuts to get a majority of the copper out of the
way, go to your files and file away the remaining copper. It seems
brutal and heavy handed but it has worked for me in the past. I did
it on copper sheet that was 18 ga. It wasn’t easy but it came out
fairly well. It wouldn’t hurt if the copper was dead soft. Just an
idea. I’m interested in hearing other responses as well.

A friend once told me to buy a deep throated, hand held fret-work
saw from a woodworkers catalog. It may have worked but the thing
looked so ungainly all I could see it doing was breaking a lot of
blades. You may want to give this a try as well.

A cutoff wheel on a hand piece might work as well but, again, be
prepared to break a lot of wheels.

Who knows? In the proper hands any of these techniques might work.
Let us know what works for you ok? I’d like to hear other options.